This invention relates to the processing of video signals, and, more particularly, to a method and apparatus for processing color video signals to improve the apparent sharpness of images having saturated colors, to improve monochrome presentations of color images, and/or to reduce the bandwidth necessary to represent color-representative signal components of video signals.
It is known that an electronic video signal (e.g. a television signal) can be encoded at a reduced bandwidth by lowering the frame refresh rate of the high spatial frequency components, while maintaining the frame refresh rate of at least a portion of the low spatial frequency components at a standard rate. If done in a specified manner, this will not cause substantial degradation in the ultimately displayed image, since human vision cannot perceive changes in high spatial resolution information at as fast a rate as it can perceive changes in low spatial resolution information. Accordingly, as has been previously set forth, an electronic video encoding and decoding system can be devised which takes advantage of this, and other, characteristics of human vision by encoding higher spatial resolution video components to be at a temporal information rate which approximately corresponds to the highest rate actually perceived by human vision for such components; thereby eliminating the need to encode these components at a higher rate, which inherently wastes bandwidth. Also, as has been previously demonstrated, the low spatial resolution information can be generated in a form which is compatible with standard television video; e.g. NTSC video. With regard to these prior teachings, reference can be made to my U.S. Pat. No. 4,517,597, and to my copending U.S. patent application Ser. Nos. 483,126, filed Apr. 8, 1983, now U.S. Pat. No. 4,628,344, and 597,482, filed Apr. 6, 1984, all assigned to the same assignee as the present application. Reference can also be made to the following publications: "Compatible Transmission of High Definition Television Using Bandwidth Reduction", W. E. Glenn, et al., Proceedings of The 37th Annual Broadcast Engineering Conference of National Association of Broadcasters, 1983; "Reduced Bandwidth Requirements For Compatible High Definition Television Transmission", W. E. Glenn, et al., Proceedings of The 38th Annual Broadcast Engineering Conference of National Association of Broadcasters, 1984.
Prior investigators have measured the response of the human visual system to isoluminance pairs. See, for example, Granger, et al., "Visual Chromaticity-Modulation Transfer Function", Journal of the Optical Society of America, Vol. 63, No. 9, September, 1973. I have also made measurements, using isoluminance chromaticity gratings, to determine human visual response. It has been observed that as the spatial frecuency of color gratings is increased beyond a certain point, the viewer can still discern the grating, but sees it as a grey grating, rather than a color grating. This phenomenon is also discussed in an article entitled "Spatial Mapping of Monkey Vl Cells With Pure Color and Luminance Stimuli", Thorell, et al., Vision Research, Vol. 24, No. 7, 1984.
It is among the objects of the present invention to utilize this phenomenon to advantage in processing color video signals, in order to achieve one or more of the following results: improvement of images obtainable from standard or high resolution color video signals by increasing the sharpness of images or portions of images having saturated colors; improvement of monochrome images derived from color signals; and reduction of the bandwidth of signals necessary for representing high resolution or standard color video signals.